Fluorite

Feldsparcleavage fragmentfrom Llano County, Texas. The two directions of goodcleavagemeet at an angle of about 90°.

The feldspars are important rock-forming minerals. You can find them inigneous rocks, such asgraniteorpegmatite, and inmetamorphic rocks, such asgneiss. They also occur as fragments insedimentary rocks, such as somesandstoneandconglomerate.

Although the feldspars can originate in other ways, they form mostly from hot magmas that cool and crystallize intoigneousrocks. These minerals occur in the rocks as grains, as cleavable masses, and as individual crystals. The crystals may be shaped like prisms, or they may be flat and slabby.

Good places to look for feldspars are in areas where granites, pegmatites, and otherintrusiveigneous rocksappear at the surface. Thepegmatiterocks of Burnet, Gillespie, Llano, and Mason counties in theLlano upliftarea of central Texas, and those of the Van Horn Mountains in Hudspeth and Culberson counties in west Texas, are especially good sources offeldspar. Large cleavable masses and crystals that are more than a foot long are found in some of these rocks.

The feldspars have a number of uses. Some of thepegmatitefeldspars from Llano County in central Texas have been crushedand used asgranulesfor built-up and composition roofs. In addition, some have been shipped to Mexico for glass-making. Some of the other uses offeldsparare in making porcelain, ceramic glazes, and scouring compounds. A few of the feldspar minerals, such as the variety ofmicroclineknown asamazonstone, are used as gemstones.

Microclinefeldsparcrystals from nearGraniteShoals Lake, Llano County, Texas.

Fibrous Gypsum.SeeGypsum.

Flint.SeeQuartz.

Fluoriteis calcium fluoride. The fluorite that is mined and sent to market, however, commonly is found mixed withquartz,calcite,limestone, or other rocks and minerals. Industry calls this mixturefluorspar.

Fluoriteis atransparenttotranslucentmineral that has a glassy luster. It may be colorless, or it may be white, pink, green, purple, brown, or blue. Some specimens show more than one color. When you rub fluorite across astreakplate, it leaves a white streak. This mineral is not particularly hard—a pocket knife will scratch it, although a copper penny will not. Fluorite has perfectcleavagein four directions. By carefully breaking a specimen, you can obtaincleavage fragmentsthat are shaped likeoctahedrons.

Fluoriteoccurs as cleavable masses, as fine or coarse grains, and as crystals. Most of the crystals arecubes, but some may beoctahedrons,dodecahedrons, or combinations of these.

Fluoritehas been found both in west Texas and in central Texas. In theLlano upliftarea of central Texas, it occurs in a number ofPrecambriangranite,pegmatite,schist, andgneissrocks. The most important, although small, deposit in this area is near Spring Creek a few miles west of Burnet in Burnet County. Here, prospectors have dug holes and pits in gneiss and schist rocks and found layers of fluorite in them.

The largest knownfluoritedeposits in Texas (they are not particularly large when you compare them with the deposits in Illinois and Kentucky) are those in the Eagle Mountains of Hudspeth County. This fluorite occurs in bothigneousandsedimentaryrocks. Many years ago, probably during the late part of theTertiaryPeriod, hotmagmafar below the surface gave off liquids and gases containing fluorine. Thesefluidsmoved up through large cracks(calledfaults) inCretaceouslimestones and Tertiaryigneous rocksand deposited fluorite in them. In places, beds oflimestonehave been replaced by fluorite. Some of this west Texas fluorite has been mined and shipped to market.

Fluoritehas octahedralcleavage. The four directions of perfect cleavage can result incleavage fragmentsthat areoctahedrons.

Fluoriteis extremely important as a flux in steel-making to help the ingredients of the molten steel blend together. In addition, it combines withsulfur, phosphorus, and other unwanted substances so that they can be removed from the steel. Other important uses of fluorite are in glass-making and in the manufacture of hydrofluoric acid. This acid is used in the aluminum industry as well as in industries that make high-octane gasoline, insecticides, and refrigerants for refrigerators and freezers.

Galena, lead sulfide, is a shiny, lead-gray, metallic mineral that has aspecific gravityof 7.4 to 7.6. It is soft enough to mark paper, and it leaves a grayish-blackstreakon a streak plate. This mineral cleaves perfectly in three directions, and thecleavage fragmentshave square corners—some arecubes.

Galenaoccurs as cleavable masses, as fine or coarse grains, and as crystals, most of which arecubes. Galena commonly is associated with other minerals; for example, some of the west Texas galena either contains some silver (then calledargentiferous galena) or occurs with it. Sphalerite, a zinc mineral, is commonly found with galena.

Galenais an important mineral because it is the chief source of lead. Compounds of lead, called white lead, red lead, and litharge, are used as paint pigments. Automobile batteries contain lead plates, and tetraethyl lead is added to gasoline to keep the car’s motor from knocking. Some other uses of lead are in bullets, type metal, solder, and cable coverings.

Galenahas perfect cubiccleavage. The three directions of cleavage are at right angles to each other resulting in cubic cleavage fragments.

Galenahas been found in several areas of Texas and has been mined in central and west Texas. None, however, has been produced inrecentyears. Most of the galena mined in west Texas was obtained from silver mines, where the galena was a by-product. Some of the west Texas galena deposits are at Altuda Mountain east of Alpine in Brewster County, in the Eagle Mountains and the Quitman Mountains in Hudspeth County, and in the Chinati Mountains and the Shafter area in Presidio County. Most of the mining has beenfrom the Shafter area (this area is described withsilver mineralsonp. 90).

In central Texas, several smallgalenadeposits have been found in Blanco, Burnet, and other counties of theLlano upliftarea. Some galena has been mined at Silver Creek in northwestern Burnet County. Here, galena occurs in cracks and as scattered grains inCambrianlimestones and sandstones.

It is probable that much of thegalenain west Texas and in central Texas was formed when hotmagmaforced out solutions containing lead. These solutions moved up through cracks and other openings in the subsurface rocks and deposited the galena in them.

Small amounts ofgalena, which likely had a different origin, have been found in Fisher, Foard, Hardeman, and Young counties. A little occurs also in rocks associated withsaltin a number of theGulf Coastal Plainsalt domes.

Garnetis not one mineral but is the name given to a group of several minerals that are very much alike. In fact, it often is impossible to tell some of them apart without using special laboratory tests.

Garnetcrystal forms include: A, trapezohedron; B,dodecahedron; C and D, combination trapezohedron and dodecahedron.

Thegarnetminerals have glassy to resinous lusters and aretransparentortranslucent. A pocket knife will not scratch them, and some specimens are too hard even forquartzto scratch. Two of the garnet minerals most commonly found in Texas arealmandite, an iron-aluminum silicate, andgrossularite, a calcium-aluminium silicate. Almandite has a deep-red or a brownish-red color. Grossularite is pale green, brownish yellow, cinnamon brown, or rose red.

Garnetminerals occur as crystals and as masses that are scattered through some of themetamorphicandigneousrocks. After they have weathered out of these rocks, the garnets make up a part of many sands and sandstones. Because these minerals so commonly occur as crystals, it is helpful to learn to recognize the crystal shapes.

Garnetminerals are found in theigneousandmetamorphic rocksof both central Texas and west Texas. In central Texas, they occur in ancientPrecambrianschistandpegmatiterocks of the Llano uplift area. Some of these central Texas garnet localities are in northeastern Mason County, central and northwestern Llano County, west-central Burnet County, and northeastern Gillespie County.

In west Texas, garnets occur inmetamorphic rocksin the Quitman Mountains, which are southwest of Sierra Blanca in Hudspeth County, and in theMicaMine area, which is south of Van Horn near the Hudspeth-Culberson County line. Garnets also have been found inigneous rocksin the Franklin Mountains a few miles north of El Paso in El Paso County.

Garnets that are found inmetamorphic rockssuch as schists were formed when great forces squeezed and heated rocks far below the earth’s surface. This heat and pressure causedelementsin the rocks to join together into different combinations to form new minerals, such as garnets. Garnets that occur scattered throughigneousrocks, such as some pegmatites and granites, cooled and crystallized from hot, igneousfluidswhen the rocks themselves formed.

Most Texas garnets are nottransparent. A few, however, are clear enough to be used as gemstones. These can be cut, polished,and mounted in rings, brooches, bracelets, and earrings. Although somegarnetis widely used as an abrasive, none from Texas has been produced for this purpose.

Gneissis ametamorphic rockthat has parallel layers or bands. Some gneiss is made up of the same minerals (chieflyfeldsparandquartz) asgranite, and it is then calledgranite gneiss. Several of the other kinds of gneiss are known asmicagneiss,conglomerategneiss,gabbro gneiss, andhornblende gneiss. In order to be a gneiss, a metamorphic rock has to have bands or layers. These bands may be either straight or wavy and either wide or narrow. In most gneisses, you will find a layer made up of long or flat mineral grains next to a layer made up of the grains of an entirely different mineral. The bands may show color differences, too. For example, a pink layer made up of feldspar grains may be found next to a black layer made up of hornblende grains. The mineral grains interlock as they do inigneous rocks, and they are generally large enough to be seen without a magnifying glass.

Gneissfrom Blanco County, Texas, showing light and dark bands.

Gneisscan form from anigneousrock, such asgranite, or from asedimentaryrock, such assandstone. Heat,fluids, and pressures below the earth’s surface change these rocks into gneiss.

Gneissthat formed duringPrecambriantime is now seen at the surface in both west Texas and central Texas. In west Texas, it occurs principally in the Van Horn area of Culberson and Hudspeth counties. In central Texas, it is found in Blanco, Burnet, Gillespie, Llano, and Mason counties of theLlano upliftarea.

One of theLlano upliftrocks is called the Valley SpringGneiss. It generally has a light color (much of it is pinkish), and it is believed to have once been asandstone. Another gneiss of this area, the Big Branch Gneiss, which has a medium to dark gray color, occurs in northern Gillespie and Blanco counties and is an alteredigneousrock. Some of the Texas gneiss rocks are suitable for use as building stones.

Goldcommonly occurs in nature as a singleelement—gold—but much native gold has a small amount of some other element, such as silver, copper, or iron, mixed with it.

Nativegoldis a shiny, yellow, metallic mineral that does not tarnish, and it leavesa shiny, golden-yellowstreakwhen you rub it across a streak plate. If silver is present, the color and streak have a lighter shade. Pure gold is extremely heavy—itsspecific gravityis 19.3. Because it is malleable, this mineral will flatten into a thin sheet when hammered. It is ductile enough to be drawn out into wires. Gold is also soft—a pocket knife will scratch it easily. When it is to be used for ornaments and jewelry, gold is usually mixed with other metal, such as silver, copper, nickel, or palladium, to make it harder. The amount of gold that is present is then indicated bycarats(orkarats). Pure gold is 24 carats. If you have a gold ring that has14 Kstamped inside it, you know that it is made of a mixture of 14 parts gold and 10 parts of other metal.

Goldcommonly occurs in nature as plates, scales, or grains. Some of the grains are large enough to be callednuggets. It also is found in a wire-like shape described asfiliform, it occurs in a network, calledreticulate, and it can have a branching and fern-like shape, described asdendritic. Gold is not often found as individual crystals.

Several other minerals, such aspyrite,chalcopyrite, andmica, are sometimes mistaken forgold. None of these, however, is malleable and ductile, and none is nearly as heavy as gold. Pyrite and chalcopyrite have dark-colored streaks unlike that of gold. Mica cleaves so perfectly that it can be split into thin, flat sheets, but gold has nocleavageat all.

The best places to look forgoldare in areas nearigneous rocksand along the creeks and rivers that drain these areas. It is thought that most gold originally was carried up from moltenigneousrock by hot solutions. The solutions moved into cracks and other openings in nearby rocks and deposited the gold, commonly along withquartz. Later, some of these gold-bearing rocks weathered away. The gold that the rocks contained either remained at the spot or was washed into creeks and rivers. These transported accumulations of loose gold are calledplacer deposits.

Placergoldin very small quantities has been found in some of the streamgravelsof Texas.

No really importantgolddeposit has ever been found in Texas, although traces and small amounts have been reported in several areas. A little gold has been found in theLlano upliftarea of central Texas. It occurs inquartzveinlets that cut through some of thePrecambrianmetamorphic rocksof Llano, Mason, northeastern Gillespie, and west-central Burnet counties. Many years ago, a small amount of gold was mined northeast of Llano in Llano County from the Heath mine. Some gold also has been found in sands andgravelsalong streams, such as along Sandy Creek and its tributaries, in parts of this Llano uplift area.

In the Trans-Pecos country of west Texas, small amounts ofgoldhave been found in the Van Horn area of Culberson and Hudspeth counties, in the Quitman Mountains district of Hudspeth County, and in the country around Shafter in Presidio County. Most of the small quantity of gold that was mined in west Texas was obtained as a by-product from the Presidio mine in the Shafter district (described withsilver mineralsonp. 90).

Small amounts ofgoldhave been reported from other parts of Texas. Some of these localities are inEoceneTertiarysandstones in theGulf Coastal Plain, inCretaceouslimestones in Irion, Uvalde, and Williamson counties, and insandandgravelin Howard and Taylor counties. None of these deposits has been found to have any commercial value.

Graniteis anintrusiveigneousrock that is made up chiefly ofcrystallinegrains or crystals ofquartzand afeldsparmineral, such asorthoclaseormicrocline. Several other minerals, includingmicaand hornblende, may also be present.

All of the mineral grains ingraniteare about the same size, and you can distinguish them without using a magnifying glass. A granite may be coarse grained, medium grained, or fine grained. When you examine this rock, you will see that its grains are not cemented but interlocked like the pieces of a jigsaw puzzle. The color of granite, which is pink, red, gray, or brownish, depends chiefly on the color of itsfeldspargrains.

Most granites formed from hot, moltenmagmathat slowly cooled and hardened far below the earth’s surface. Because of this slow cooling, fairly large mineral grains were formed.

Polished section of pinkgranitefrom Gillespie County, Texas.

Granites are now seen at the surface in several areas of Texas. They were gradually uncovered as the areas became higher and the overlying rocks slowly weathered away. One of these areas is theLlano upliftof central Texas where the granites occur in Blanco, Burnet, Gillespie, Llano, and Mason counties. These granites formed duringPrecambriantime and are believedto be about a billion years old. (Scientists are now able to determine the age of some rocks accurately by very precisely measuring the relative amounts of isotopes produced by decay of radioactive minerals.)

Texas State Capitol building at Austin is made of Burnet Countygraniteobtained from Granite Mountain nearMarbleFalls, Texas.

Granites also appear at the surface in the Trans-Pecos country of west Texas. Some of these areas include the Franklin Mountains of El Paso County, the Quitman Mountains of Hudspeth County, the Chisos Mountains of Brewster County, and the Chinati Mountains of Presidio County.

Red, pink, and gray granites from quarries in theLlano upliftarea are widely used as building stones and monument stones. A large quarry atGraniteMountain just west ofMarbleFalls in Burnet County has supplied pink granite for buildings in many parts of the United States. The Texas Capitol building and several other State buildings in Austin are made of this granite.

Graphiteis a mineral that is made up of a singleelement—carbon. (Diamond, althoughit does not look at all like graphite, is acrystallineform of carbon.) Graphite is a steel-gray or black mineral that commonly has a metallic luster. It is not heavy and is extremely soft. Graphite will soil your fingers and leave a black mark on paper. This mineral cleaves perfectly in one direction and splits into thin flakes that feel greasy.

To help distinguishgraphitefrommolybdenite, a mineral it resembles, you can use a shiny, glazed surface, such as is found on a saucer or a plate, to test itsstreak. When rubbed across this kind of surface, graphite will leave a black streak, but molybdenite will leave a greenish one.

Graphitecommonly occurs as scales, as sheet-like layers, or as compact masses. It may be found mixed withclayor other impurities, and it then looks dull and earthy. Crystals of graphite, which are seldom found, are 6-sided and flat.

Graphiteoccurs in Llano, Burnet, and other counties in theLlano upliftarea of central Texas. One of the Nation’s most important graphite mines is located in the Clear Creek area several miles northwest of Burnet in Burnet County. Some graphite has also been mined near Lone Grove in Llano County. In addition, a graphiteschist, obtained south of Llano in Llano County, has been used as a filtering material.

Graphiteis used in pencil lead, generator brushes, and lubricants.

All of thisgraphiteoccurs in extremely oldPrecambriangraphiteschistrocks that we now see at the surface in this part of Texas. It is believed that the schists were once ancientsedimentary rocks, such as shales, which contained organic matter. Long ago, great forces below the earth’s surface altered these rocks. When this happened, the organic material that they contained changed into the mineral we know as graphite.

Graphitehas a number of uses. It is mixed withclayto make the pencil lead that we use for writing. It serves as a lubricant, either alone or mixed with oil, grease, or water. In addition, graphite is used to make generator brushes, stove and shoe polish, and special paints. Because it can stand great heat without melting, some graphite is mixed with clay to make the pots or crucibles that hold molten metals.

Grossularite.SeeGarnet.

Gypsite.SeeGypsum.

Gypsumis a hydrous calcium sulfate. This mineral is normally colorless or white, but impurities cause it to appear gray, brownish, yellowish, or reddish. It istransparentortranslucentand is not heavy. When you rub gypsum across astreakplate, it leaves a white streak. This mineral is so soft that a fingernail scratches it easily. Gypsum occurs in several varieties.

The colorless, glassy, andtransparentvariety ofgypsumis calledselenite. It is found as cleavable masses and as crystals that are prism-shaped or flat and diamond-shaped. It is not uncommon for two crystals to be joined together so that they have a swallow-tail shape—these crystals aretwinned. Groups of flat selenite crystals arranged together so that they resemble flowers are calledrosettes. Many of these have been found in Nolan County.

Gypsumhas four directions ofcleavage. One of these directions is so perfect that someselenitesplits into thin, clear sheets that may be mistaken formica; other selenitecleavage fragmentsmay be mistakenforcalcite. You can distinguish selenite sheets from calcite by testing their hardness (selenite is softer) and by putting a drop or two of dilute hydrochloric acid on them. The acid will fizz and bubble on calcite but not on the selenite gypsum. There is also a quick way to distinguish the thin selenite cleavage fragments from mica. After you carefully bend a thin sheet of mica, it will snap back to its original shape without breaking. Selenite gypsum, however, is not elastic. It will bend, but it will break if you try to straighten it again.

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